Aside from those OS choices around flash or hard disk options, you also have a 3rd option which is to install ESXi over the network and run it from RAM!
I have the following opinion about whether to install ESXi to flash or hard disk:
If you install to flash, there is no S.M.A.R.T. reporting to warn you of disk failure. This is a problem because when SD cards fail, they don’t just stop working and allow your ESXi host to fail over, they start to slow down and get slower and slower over a period of time, until they suddenly crash.
SD cards are cheap to buy, but heck so are small SSD hard disks when you look at the over all cost of a server, a couple of hundred bucks divided across all the users of the VMs hosted on the server is nothing.
Running your ESXi hosts in RAM would be a better option if you’re trying to get some cheap, fast storage… But then you have to have a PXE server set up to enable this.